The present invention relates to an apparatus and method for generating moisture standards of known water concentrations in gases. The method and apparatus of the invention utilize volumetric measurement of water delivered to the gas to calculate the resulting water concentration. In particular, the method and apparatus of the invention provide for delivery of a preselected amount of water vapor to a flowing gas stream, thus enabling quick and reliable calculation of the resulting concentration of water in the gas.
Measurement of low levels of moisture content in dry gases is critical for applications in which high or ultrahigh purity gases are used. For example, the measurement of ppb levels of moisture in ultrahigh purity gases used in the microelectronics or semiconductor industries is crucial. Moisture is one of the most ubiquitous and serious contaminants in the fabrication of microelectronic devices, such as wafers. Its presence in the gas phase can lead directly to impaired wafer yield. Moisture in certain gases used in fabrication of microelectronic devices can also accelerate the corrosion of tubing, regulators and valves used in handling these gases, and the corrosion products can negatively impact wafer yields and quality. Therefore, stringent measurement and control of moisture as a contaminant is required.
Instruments which can be used to measure moisture at such low levels include electrolytic cells, dielectrics of polymer/ceramic, vibration crystals, Fourier transform infrared (FTIR) spectrometers, atmospheric pressure ionization mass spectrometers, and chilled-mirror frost-point hygrometers. These instruments generally require calibration using gases with known moisture content. For example, FTIR analysis of moisture in a gas requires the generation of a calibration curve of absorbance due to water at a selected wavelength, versus known concentration of water in that gas. In order to generate such a calibration curve, gas standards with known concentrations of water are necessary. In addition, it is often necessary to check the response of a moisture meter used to monitor the moisture level in a gas, requiring a gas having a known water concentration, or xe2x80x9chumidity challenge.xe2x80x9d Moreover, measurement of the corrosion of steel as a function of the moisture contamination in hydrogen chloride gas requires the generation of gas standards having known concentrations of water vapor.
Such calibration gases are typically produced using moisture standard generators. There are currently two primary methods for generating known concentrations of water vapor in a flowing gas: permeation tubes and diffusion vials.
Permeation tubes are containers having a permeable polymeric membrane and that are filled with water. The tubes are placed in a flowing dry gas stream, and water vapor permeates through the membrane and into the dry gas. Permeation tubes operate on the principle that the amount of water permeating through the membrane is constant at a constant temperature. The resultant water vapor concentration in the flowing dry gas is determined by multiplying the molar gas constant for water vapor by the permeation rate, and dividing by the gas flow rate. To produce standards having different moisture concentrations, either tubes having membranes with different emissivities must be employed, or the flow rate of the gas must be varied.
Permeation tubes are the most widely employed moisture standard generators, perhaps because of their convenience. However, problems with the reliability of these devices often occur. One problem is that the moisture emitted from the permeation tube is highly dependent on the control of temperature and pressure. Thus, slight variations in the temperature or pressure of the flowing dry gas will cause the resultant moisture content of the gas to vary significantly.
A potentially more serious problem with permeation tubes is the deviation of the tubes from their claimed accuracy. In experiments or calibrations using moisture standards, complex ancillary equipment is often used, making it difficult to ascribe irregularities that may arise in the measurements to a particular causative agent. Therefore, the moisture standard generator itself should be as reliable as possible. Although initial calibration of the manufacturer""s equipment is performed with National Institute of Standards and Technology (NIST) traceable standards, a user employing a particular permeation tube must depend upon the continued validity of the manufacturer""s calibration. It has been shown that permeation tubes may be in error of their reported values by as much as about 30% around 1 ppm when compared to NIST humidity standards validated by an optical frost-point hygrometer. See Huang, Peter A., xe2x80x9cAccuracy of PPM Humidity Standard Based on Permeation Method,xe2x80x9d Proceedings of the Sixth International Meeting on Chemical Sensors, National Institute of Standards and Technology, Gaithersburg, Md. (Jul. 22-25, 1996).
If the reliability of permeation tubes is suspect, calibration may be verified using commercially available moisture meters. However, these meters are expensive and are themselves often inaccurate, unreliable and responsive over only narrow concentration ranges. Another recourse is to send the tube back to the manufacturer for re-calibration, but this is very time consuming and inconvenient. A third alternative is that the calibration could be performed in-house. However, this would be impractical and extremely inconvenient, as the calibration would require NIST traceable standards, proper equipment and experienced personnel.
Diffusion vials offer a significant increase in certainty at the expense of convenience and flexibility. A diffusion vial system consists of a bottle of water having a small hole at the top and contained in a temperature-controlled housing. The water is introduced into an inert dry gas by flowing the gas over the vial opening. The system operates on the principle that the vapor pressure of water, and therefore the rate of release of water vapor from the vial, is constant at a constant water temperature. Thus, once the system reaches a steady temperature and constant flow rate, the rate of water introduced into the dry gas is theoretically constant.
The main advantage of this system is that the total amount of water introduced into the dry gas can be accounted for gravimetrically, i.e. the exact quantity of water introduced into the flowing gas stream can be determined by weighing the vial before and after use. However, a significant problem with the diffusion vial is that it is impractical to obtain values of the water concentration before the end of the experiment for two reasons. First, obtaining water consumption rates essentially requires stopping the experiment because one must stop the gas flow, disconnect the gas lines, remove the vial and weigh it. Hence, any measurement of concentration will necessarily be performed at the end of the experiment, making intermediate water consumption rates impossible to obtain. Second, the diffusion vial is sensitive to temperature, pressure and water level, and disturbing these conditions by stopping the gas flow and disconnecting the gas lines will affect the rate of diffusion. Therefore, the diffusion vial method of generating moisture standards is more suited to providing an accurate average, rather than instantaneous, water concentration.
Another problem with the diffusion vial system is that this method is useful only over a relatively high concentration range, i.e. thousands of ppm. To achieve lower concentrations of water, one could attempt to pass higher flow rates of gas over the vial, but this leads to temperature control problems. Another solution is to dilute the highly concentrated vapor and reject the excess flow. However, this requires dilution apparatus and flow meters, each having uncertainties of their own, and these errors would combine additively thereby diminishing the accuracy of the moisture concentration.
U.S. Pat. No. 3,592,182 to Richardson discloses an apparatus for humidifying air flowing through a furnace duct for room use. The humidifier is designed for installation within the confines of a horizontal furnace duct. The humidifier comprises a hollow shell which is constructed to provide a container for a predetermined amount of water. The water level in this container is maintained by a float valve. A sponge is positioned within the hollow shell. As the air stream from the furnace blower flows across the moistened sponge, it is humidified for room use.
The humidifier disclosed by Richardson is unsuitable for use as a moisture standard generator for several reasons. Most significantly, this humidifier has no means for quickly and unobtrusively varying the amount of water introduced into the air stream. The water in the hollow shell is maintained at a constant level by a float valve, and this level cannot be changed without stopping the air flow, dismantling the air duct, and changing the position of the float valve. The rate of water delivery to the sponge is always equal to the maximum rate of evaporation from the sponge, which is determined by the temperature and flow rate of the air stream. Moreover, very low levels of moisture, i.e. the ppb levels of interest in microelectronics or semiconductor industries, cannot be generated with this humidifier. In addition, the humidifier of Richardson can be used only in a horizontal portion of a duct.
Accordingly, there is a need in the art for a moisture standard generator capable of introducing a constant, preselected amount of moisture into a dry matrix gas, in which the amount of moisture can be varied easily, reliably and quickly, without the use of complicated dilution equipment. There is also a need in the art for a moisture standard generator which is not sensitive to temperature, and which is suitable for operation at ambient temperature and pressure.
The present invention obviates many of the problems associated with conventional moisture standard generators by providing an apparatus and method for introducing a known amount of water vapor into a gas, which uses volumetric measurements to determine the amount of water introduced, rather than the gravimetric measurements relied on by diffusion vials, or the manufacturer-dependent emissivity rates of permeation tubes. Thus, the present invention relates, in part, to an apparatus for introducing a preselected amount of water vapor into a flowing gas stream at a constant rate, which comprises: (a) a syringe comprising a housing and a plunger, and having a needle attached thereto, (b) an evaporator, said evaporator being attached to the end of the needle, wherein the evaporator may be situated in a flowing gas stream, and (c) a means for moving the plunger of the syringe at a constant speed. The water to be used in generation of the moisture standard is contained within the syringe housing. Pressure is applied to the syringe to move the syringe plunger at a constant speed such that water is transferred from the syringe into the evaporator at a constant rate. The water introduced into the evaporator simultaneously evaporates at a constant rate into the flowing gas stream, thereby generating a moisture standard with a known concentration of water vapor.
Accordingly, in one embodiment, the present invention relates to an apparatus for introducing a preselected amount of water vapor into a flowing gas stream at a constant rate, which comprises: (a) a syringe having a needle attached thereto; (b) an evaporator, said evaporator being attached to one end of the needle, wherein the evaporator may be located in the flowing gas stream; and (c) a means for applying pressure to the syringe, such that water may be delivered at a constant rate from the syringe through the needle into the evaporator. Pressure is applied to the syringe such that the plunger of the syringe moves at a constant rate, thereby dispensing liquid from the syringe to the evaporator at a constant rate.
The present invention also encompasses a method for introducing a preselected amount of water vapor into a flowing gas stream at a constant rate, which comprises: (a) providing a syringe having a needle attached thereto, wherein the syringe contains liquid water; (b) applying pressure to the syringe, such that an amount of water is transferred from the syringe through the needle into an evaporator at a constant rate, said evaporator being attached to one end of the needle, and situated in the flowing gas stream; and (c) allowing the water to evaporate from the evaporator into the flowing gas stream. In a preferred embodiment, the evaporator is a piece of material, such as felt, which is attached to one end of the needle. The other end of the needle is attached to the syringe.
In addition to water, many other liquids may be used in the apparatus and method of the present invention. Thus, the present invention also relates to an apparatus for introducing a preselected amount of vaporized liquid into a flowing gas stream at a constant rate, which comprises: (a) a syringe having a needle attached thereto; (b) an evaporator, said evaporator being attached to one end of said needle, wherein the evaporator may be located in the flowing gas stream; and (c) a means for applying pressure to the syringe, such that the liquid to be vaporized may be delivered at a constant rate from the syringe through the needle into the evaporator.
The present invention also relates to a method for introducing a preselected amount of vaporized liquid into a flowing gas stream at a constant rate, which comprises: (a) providing a syringe having a needle attached thereto, wherein the syringe contains the liquid to be vaporized; (b) applying pressure to the syringe, such that an amount of the liquid to be vaporized is transferred from the syringe through the needle at a constant rate into an evaporator, said evaporator being attached to one end of the needle, and situated in the flowing gas stream; and (c) allowing the liquid to evaporate from the evaporator into the flowing gas stream; wherein the amount of vaporized liquid is selected to result in a preselected concentration of the vaporized liquid in the flowing gas stream.
The apparatus and method of the present invention enable the user to determine the resulting concentration of moisture or other vaporized liquid in the gas quickly and conveniently, without stopping the flow of gas and opening the gas delivery system. When necessary the syringe can be easily disconnected from the needle, removed from the apparatus, refilled with water or other liquid and reconnected (or replaced with a full syringe), with only minimal disturbance to the system. Similarly, the syringe can be disconnected and replaced with a syringe having a larger or smaller volume, thereby permitting the concentration of moisture or other vaporized liquid to be varied easily, reliably and quickly without interrupting the gas flow. By selecting the appropriate size syringe, rate of syringe compression, and the flow rate of the gas stream, a wide range of moisture concentrations or concentrations of other vaporized liquids can be achieved with the apparatus and method of the present invention, without the use of complicated dilution apparatus, and with minimal disturbance to gas handling systems.